allow-prereleases: restrict to alpha, beta, rc

I maintain a C++/Python package that uses the Python's C-API. I'd like to test the package against pre-releases, but only after the ABI/API have stabilized, i.e., RCs.

Right now, setup-python has allow-prereleases which falls back to the most recent pre-release if there's no stable release yet.

I'd like to propose a more fine-grained control over which prereleases should be selected:

• (allow-prereleases: 'dev': Use dev, alpha, beta, rc prereleases)

• allow-prereleases: 'alpha': Use alpha, beta, rc prereleases (not dev, equivalent to allow-prereleases: true)

• allow-prereleases: 'beta': Use beta, rc prereleases (not dev, not alpha)

• allow-prereleases: 'rc': Use rc prereleases (not dev, not alpha, not beta)

Even simpler, one could use allow-releases:

• (allow-releases: 'dev': Use dev, alpha, beta, rc, stable releases)

• allow-releases: 'alpha': Use alpha, beta, rc, stable releases (not dev, equivalent to allow-prereleases: true)

• allow-releases: 'beta': Use beta, rc, stable releases (not dev, not alpha)

• allow-releases: 'rc': Use rc, stable releases (not dev, not alpha, not beta)

• allow-releases: 'stable' (default): Use stable releases (not dev, not alpha, not beta, not rc)

Start by tracing how setup-python currently interprets allow-prereleases and selects Python versions. Compare the proposed prerelease levels, resolve whether allow-prereleases or allow-releases is the intended interface, and verify that each level selects the stated versions without changing the default stable behavior.